potm is alot harder than totm. With potm as already stated its best to have a focal point and make sure its only one so the person viewing doesnt get lost in the photo.
Try and stick to one object, be it one/group fish/shrimp/snail (same species would be best) or perhaps a plant.
The 2nd step is to make sure the object is in the right location in the photograph (side/center)
Next step is ofcourse getting a clear shot. Make sure theres almost no blur and the object is as crisp as it can be. Clean the camera lens and the tank glass/ provide extra lighting and then take your photograph (more like 200 photographs)
other tip is if you are trying to take a close up photo of an object, never move your camera close to the aquarium, instead either zoom in slightly (if zooming best to have a tripod), once you zoomed in slightly take the photograph, you can always crop the imagine to get rid of the unnecessary areas..
try and take a photograph of the fish in action or in a pose thats makes one go "wow", a simple shot of a fish going for food or looking at the camera seems boring.
Also take the photo with a high mega pixel camera, it helps alot when you start cropping.